Low-grade intra-articular inflammation promotes symptoms and disease progression in osteoarthritis (OA). The multiligand Receptor for Advanced Glycation Endproducts (RAGE) is implicated in several extraskeletal degenerative disorders linked to low-grade tissue inflammation and aging, including diabetes mellitus and Alzheimer's Disease. RAGE ligands include calgranulin/S100 proteins that can exert inflammatory cytokine-like effects. Articular chondrocytes express RAGE, and expression of RAGE and S100A11 are up-regulated in OA cartilage. S100A11 induces chondrocyte hypertrophy. Moreover, CXCL8 and TNFa induce S100A11 expression and chondrocyte hypertrophy, and S100A11-, CXCL8-, and TNFa-induced chondrocyte hypertrophy are RAGE-dependent. Our objective is to define how the prototypical chondrocyte calgranulin S100A11 modulates chondrocyte differentiation and function in vitro via RAGE signaling, and to define the role of RAGE in pathogenesis of OA in vivo. We specifically aim to: 1) Define the molecular structural requirements by which S100A11 modulates chondrocyte differentiation and function. We will test the hypothesis that S100A11 induces hypertrophic differentiation, MMP-13 and ADAMts5 expression, and proteoglycans loss in chondrocytes in a manner that requires one or both S100A11 calcium binding EF hands, as well as multimerization by transglutaminase 2 (TG2). 2) Define the mechanism by RAGE signaling transduces S100A11 effects on chondrocyte differentiation and function. We will test the hypothesis that S100A11 induces RAGE-specific signaling that promotes Rac1 and PI3K signaling upstream to MKK3 and p38 MARK activation, followed by essential downstream HDAC4 depletion to induce terminal hypertrophic chondrocyte differentiation. 3) Test the hypothesis that S100A11 and RAGE directly mediate the pathogenesis of murine OA in vivo by assessing OA severity in a knee instability model in RAGE-/- vs. congenic RAGE+/+ mice. Also, we will test RAGE-/- mouse femoral head cartilage explants for reduced S100A11 and inflammatory cytokine-induced MMP-13 and ADAMts5 expression, chondrocyte hypertrophy, and proteoglycans loss relative to RAGE+/+ explants ex vivo. Completion of the work will reveal new potential targets for therapeutic intervention in OA by disconnecting inflammation from deleterious alteration of chondrocyte differentiation and function.